Currently, a non-aqueous electrolyte secondary battery including a lithium ion secondary battery that is utilized for a mobile device such as a mobile phone is available as a commercial product. The non-aqueous electrolyte secondary battery generally has a configuration in which a positive electrode having a positive electrode active material or the like applied to a current collector and a negative electrode having a negative electrode active material or the like applied to a current collector are connected to each other via an electrolyte layer in which a non-aqueous electrolyte solution or a non-aqueous electrolyte gel is held in a separator. Further, charge and discharge reactions of a battery occur as ions such as lithium ions are absorbed into and desorbed from an electrode active material.
Incidentally, in recent years, it has been desired to reduce the amount of carbon dioxide in order to cope with global warming. Hence, a non-aqueous electrolyte secondary battery having a small environmental burden has been used not only in a mobile device or the like but also in a power source device of an electrically driven vehicle such as a hybrid vehicle (HEV), an electric vehicle (EV), or a fuel cell vehicle.
A non-aqueous electrolyte secondary battery directed to the application to electrically driven vehicles is desired to have a high output and a high capacity. According to this, an amount of chemical energy to be stored in the battery increases, so that safety securement becomes more important. Further, the non-aqueous electrolyte secondary battery directed to the application to electrically driven vehicles is desired to have cycle characteristics that the capacity can be maintained even when the charge and discharge cycle is repeated for a long period of time.
Herein, in the related art, there have also been a large number of proposals of a separator used in a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. For example, WO 2007/116672 A discloses a polyolefin microporous film including: a laminate having three or more layers including two surface layers and at least one intermediate layer, in which a limiting viscosity [η] and a pore closing temperature of each of the surface layers and the intermediate layer are controlled to predetermined values, respectively. Based on the literature, with such a configuration, it is possible to obtain a polyolefin microporous film in which safety at the time of overheating is maintained and a mechanical strength is satisfactory as well.